Short introduction on SU-8 photoresist spin coating

The coat of SU-8 photoresist on a substrate can be done by several techniques. The most used is spin coating which consists in putting a puddle of SU-8 photoresist on a rotated substrate. The rotation speed, the acceleration and the SU-8 photoresist viscosity will define the thickness of the SU-8 photoresist layer.

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Using a spin coater, contrary to the other ones, use a lot more SU-8 photoresist than necessary, because 95% of the SU-8 photoresist is lost, ejected during the rotation. However, it is the most used technique in laboratory, because it is easy to use and really repeatable.

The success of this step is of course to really match the equipment chosen but not only, we are going to give you here some tricks to succeed or improve your coating.



The substrate wettability

Doesn’t matter the substrate where the SU-8 photoresist is coated, the SU-8 has to be spread on the surface, not to retract during the bakes, and stay on the surface of the substrate at the end of the process. For that, the SU-8 photoresist has to wet enough the substrate. It exists several ways to ensure a good wettability such as a good cleaning of the wafer, a good dehydration, some plasma treatment and the use of adhesion promoter in solid or liquid phase… A measure of the angle contact with water can be a good indicator on the success of the future coat. Indeed, for example with the resin SU-8 3000 coated on a PET film (in the aim to create dry film on soft substrate) the contact angle as to be 80°, really precisely. If the angle is higher, the resin will not spread normally on the surface and with a lower angle, the SU-8 photoresist will stick on the PET without being possible to be peeled off.

The SU-8 photoresist viscosity

The different SU-8 photoresist have their own viscosity, this wide range enables to obtain a lot of different thicknesses of the layer going from some hundreds of nanometers to hundreds of micrometers by coating. The viscosity has a great influence on the thickness of the layer but the viscosity change with environmental parameters (temperature, humidity) and with time.
The viscosity depends on the temperature and thus if the other parameters are constant, the thickness of the layer is function of the temperature. All parameters (rotated speed, acceleration…) have to be adjusted for the working temperature. After that the temperature has to be regularly measured to make sure it remains constant. We advise you to use the SU-8 photoresist at room temperature, for that you have to wait at least 2 hours for a bottle of 500mL of SU-8 resin for example.
The viscosity change also with the quantity of solvent inside the SU-8 photoresist. Solvents are volatile so the percentage of solvent changes with time. That is why the process can change if the SU-8 photoresist is too old or has been stocked in a wrong manner.
At last, when the SU-8 photoresist is used, the solvents are evaporating in the air and the process timing has to be respected. What’s more, the evaporation will be more or less high according to the air humidity. So, such as the temperature, some adjustments can be done to adapt the process to the room humidity.


The SU-8 photoresist dispensing

The SU-8 photoresist volume and the way it put on the substrate are two parameters that define the coating. The quantity of resin to depose depends on the kind of resin but for SU-8 , it is, at least, 1ml of resin by inch of the wafer.
The coating can be done by several ways. The simplest is to directly pour the SU-8 photoresist from the container but the quantity is unknown and it is not always handy. It is also possible to use a syringe or a micropipette of 10 ml.

The use of micropipette to pour the SU-8 photoresist

The use of 10ml micropipette enables to depose a well-known volume and to minimize the loss.
According to the viscosity of the SU-8 photoresist used, it can be necessary to cut the end of the tip to make the filling easier.
What’s more a part of the SU-8 photoresist will stay inside the tip always because of the viscosity so the volume has to be over estimated and thus a volume of 5 at 6 ml for a 4inch wafer gives good results.


Dynamic SU-8 spin coating

For really viscous resin such has the SU-8 ones for the brand higher than 50µm, it is interesting to depose the SU-8 photoresist on a dynamic manner, it means when the wafer is rotating. This technique enables a better spread of the resin on the surface than the static coating.
The dynamic coating is done at low speed (500rpm) and low acceleration (100rpm/s) during a time around the 30 seconds.


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